This study used data from the School Nutrition Dietary Assessment III Study to examine the dietary patterns of school meal program participants and nonparticipants and the relationship between school meal participation and children’s Body Mass Index (BMI). School Breakfast Program (SBP) participants ate more low-nutrient energydense (LNED) baked goods and more calories at breakfast than did nonparticipants. National School Lunch Program (NSLP) participants had lower intake of sugar-sweetened beverages and a lower percentage of calories from LNED foods and beverages than did nonparticipants. Overall, NSLP participation was not significantly related to students’ BMI, although participants were less likely to be overweight or obese than nonparticipants among Black students but more likely to be so among “other race” students. SBP participants had significantly lower BMI than did nonparticipants, possibly because SBP participants are more likely to eat breakfast and eat more at breakfast, spreading calorie intake more evenly over the course of the day.